Artificial Ground Freezing for TBM Break-through - Design ...

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1 INTRODUCTION

Contract No. DC/2009/05 is one of the HATS Stage 2A contracts being implemented by the Government of

the Hong Kong Special Administrative Region to improve the water quality in the Victoria Harbour. This

construction contract was awarded by the Drainage Services Department to a joint venture of China State

Construction Engineering (Hong Kong) Limited and Shanghai Tunnel Engineering Company Limited

(CSSTJV). Works under this contract consist of

construction of an interconnection tunnel, and of a

diaphragm-walled cofferdam for the main pumping

station at Stonecutters Island Sewage Treatment Works.

Hyder Consulting Ltd. was appointed by CSSTJV to

carry out detailed design for the construction of the 4m

diameter interconnection tunnel which comprises Part

A Tunnel 236m in length excavated by TBM, and Part

B Tunnel 14m in length excavated by hand-mining.

AGF was employed as the ground improvement

method to facilitate TBM break-through from the

launching shaft. Figure 1 shows the site layout.

This paper presents the detailed design, covering

both the thermal and stress analyses, and the required

laboratory testing for the application of AGF using

ABSTRACT

Artificial ground freezing (AGF) has been widely adopted in Shanghai, China, as the ground

improvement method for break-through of tunnel boring machines (TBM) from their launching

shafts and into their receiving shafts. In Hong Kong, AGF application has in the past been limited

to the construction of mined adits and cross-passages between tunnel bores. In one of the

construction contracts under the Harbour Area Treatment Scheme (HATS) Stage 2A, the

contractor has initiated to adopt AGF for the first time in Hong Kong using brine for TBM break-

through. Under the contract, a tunnel about 4m in diameter and 250m in length is to be

constructed inside Stonecutters Island Sewage Treatment Works at 30m below ground in marine

deposits, alluvium and decomposed granite by TBM to connect a new pumping station to the

existing pumping station. This paper presents the design considerations for the application of

AGF using brine for TBM break-through. It details the thermal and stress analyses required to

confirm the viability of the construction method, and the laboratory testing required for derivation

of the necessary thermal and geotechnical parameters of the soils.

ARTIFICIAL GROUND FREEZING FOR TBM

BREAK-THROUGH – DESIGN CONSIDERATIONS

R.K.Y. Leung & K.K.Y. Ko Hyder Consulting Ltd., Hong Kong

H.B. Hu China State – Shanghai Tunnel Joint Venture, Hong Kong

A.K.K. Cheung Ove Arup & Partners Hong Kong Ltd., Hong Kong

W.L. Chan Drainage Services Department, HKSAR Government

Figure 1: Site Location Plan

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Figure 2: Geological profile

brine for TBM break-through. Thermal analysis was carried out to estimate the freezing energy and time

needed to achieve a frozen zone down to a designated temperature. As some of the vertical freezing pipes had

to be lifted prior to TBM break-through, an assessment was also made of the temperature change with time

after those pipes were removed. Stress analysis was carried out to confirm stability of the soil mass with the

soft-eye cut out in the diaphragm-wall of the launching shaft. Assessment of the effects of frost heave and

thaw consolidation with the help of numerical modeling

is also discussed. 2 SITE GEOLOGY

The site is formed by reclamation with a ground level of

about +5.5mPD. The interconnection tunnel is situated at

approximately 30m below ground level and the

encountered geology at TBM break-through from the

launching shaft includes Marine Deposits and Alluvium,

as shown in Figure 2 and described below:

Marine Deposits - firm to stiff, slightly sandy silty

CLAY with occasional angular to subangular fine gravel

sized rock and shell fragments;

Alluvium - medium dense to dense, clayey silty fine to

coarse SAND or stiff to very stiff, sandy silty CLAY,

with some subangular to subrounded fine to medium

gravel sized rock fragments.

The design groundwater level is at 2m below existing

ground level.

3 DESIGN CONSIDERATION

General design considerations for AGF are as follows:

Groundwater Level – Soil with sufficient moisture

content is a pre-requisite of AGF. In this project, with

the lowest groundwater level at +1.0mPD and tunnel

crown level at -18.0mPD, the tunnel is completely

submerged and hence the soil around it is saturated;

Soil Material – AGF is considered generally effective in improving the strength of silty, clayey and sandy

type of soil materials but less effective for bouldery soil though cut-off effect would still be achieved. The

soil materials encountered at the locations where AGF was applied vary from clayey, silty to sandy;

Salinity – Salinity affects the freezing point of water and hence the saturated soil materials. It also affects the

mechanical properties of the frozen soil. Laboratory testing has been carried out on frozen in situ soil samples

to determine the thermal and mechanical properties of the soils;

Groundwater Flow – Groundwater flow affects the shape and time for formation of the frozen soil. In

significant groundwater flow, more energy will be required to create the design frozen soil block.

Groundwater flow has been monitored through the reading of piezometers in this case and found insignificant;

Frost Heave and Thaw Consolidation – With the existence of some sensitive building structures along the

tunnel alignment, frost heave and thaw consolidation needed to be considered. In view of this, laboratory

testing to determine frost heave and thaw consolidation ratios has been carried out for the heave and

settlement assessment.

4 LABORATORY TESTING

Laboratory tests were performed on soil samples retrieved from the site to obtain the mechanical properties of

frozen soils and thermal properties of soils in both the non-frozen and frozen states. The tests were carried out

in accordance with the Chinese national code GB/T50123-1999 Standard for Soil Test Method (土工试验方

法 ) and Chinese code for coal mining industry MT/T593-1999 Testing for Physical and Mechanical

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Figure 3: Typical stress-strain curve of unconfined compression test

Figure 4: Typical strain-time & strain rate-time plots of creep test

Properties of Artificially Frozen Soil (人工冻土物理力学性能试验). Table 1 gives a summary of the tests

performed.

Table 1: Summary of laboratory testing

Test Details

Unconfined compression 2 tests each at -10ºC, -15ºCand -20ºC for each soil type

Creep For each soil type, one test each for each combination of temperature

(-10ºC, -15ºC, -20ºC) and stress level (0.3q, 0.4q, 0.5q, 0.7q)*

Freezing temperature 2 tests for each soil type

Frost heave 2 tests for each soil type

Thaw settlement 2 tests for each soil type

Thermal conductivity One test each at 20ºC, 0ºC, -5 ºC and -20ºC for each soil type

Specific heat capacity One test each at 20ºCand -10ºC for each soil type

*q is the peak axial stress or axial stress at axial strain of 20% if there is no peak.

The unconfined compression tests

were carried out with specimens cut from

Mazier soil samples. Compression was

applied at a rate of 1% strain/min. until an

axial strain of 20%. Figure 3 shows a

typical stress-strain curve of the test.

Whilst the test standard defines the

strength as the peak stress or the stress

corresponding to 20% strain if no peak is

experienced, a more conservative

approach of taking the stress at which the

specimen began to yield as the strength

was adopted in arriving at the design

values.

In the creep test, compression was

applied at a rate of 1% strain/min to the

required stress level and then maintained

for 24 hours. Figure 4 shows a typical

plot of the creep strain and strain rate

against time. The tests results showed

that at a stress level of 0.5q or lower, the

strain of the tested specimens consistently

came to be stable with time. Hence, 50%

the design compressive strength is taken

as the design creep strength of the soils.

The creep modulus corresponds to the

ration of the applied stress to the strain

when stable.

In the tests for frost heave ratio,

temperature of soil specimens were

brought down to below their freezing

temperature and the change in height of

the specimens was measured. Likewise,

in the test for thaw consolidation ratio,

frozen soil specimens were allowed to

thaw and the reduction in height of the

specimens was measured.

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Tables 2 and 3 summarize the values of the mechanical and thermal properties adopted in the design.

Table 2: Design values of mechanical properties

Soil UCS*

(MPa)

Young’s

Modulus E50

(MPa)

Creep

Strength

(MPa)

Creep

Modulus

(MPa)

Frost Heave

Ratio

Thaw

Consolidation

Ratio

Frozen Marine Deposits 2.0 160 1.0 21 1.18% 13.00%

Frozen Alluvium 3.5 315 1.75 36 5.10% 8.65%

* at temperature of -15°C

Table 3: Design values of thermal properties

Soil Freezing

Temperature

(°C)

Specific heat capacity

kJ/(kg °C)

Thermal Conductivity

W/(m K)

In-Situ Frozen -20°C -5°C 0°C 20°C

Marine Deposits -1.71 2.14 1.28 1.966 1.628 1.485 1.367

Alluvium -1.84 1.48 1.04 2.272 1.993 1.916 1.746

5 CONSTRUCTION ASPECTS AFFECTING THE DESIGN

Figure 5 shows the configuration of the ground freezing work for TBM launching. Key aspects affecting the

design are discussed as follows.

Figure 5: Configuration of the Frozen Soil Wall for TBM Launching

Use of brine as freezing agent Two artificial ground freezing systems are available in the construction industry, using either liquid

nitrogen or brine. In this project, a closed circuit freezing system using brine was adopted. Brine lowered to a

temperature of -28°C using industrial refrigeration plant was circulated into freezing pipes installed in the

ground in a regular pattern. The target was to form a 2.5m thick frozen block with a temperature of -16°C in

front of the TBM break-through.

Partial removal of diaphragm wall prior to TBM launching

It was intended to remove the part of the diaphragm wall of the launching shaft facing the TBM prior to TBM

launching for facilitating the launching operation. The frozen soil block thus should have the capability of

withstanding the soil and water pressure acting from the back of it.

Partial extraction of ground freezing pipes prior to TBM launching

Prior to TBM launching, the ground freezing pipes falling within the drive of the TBM had to be partially

extracted to above the tunnel crown level in order not to obstruct with the TBM drive. After the pipe

extraction, the frozen block had to remain frozen for at least 72 hours to allow the TBM launching operation.

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6 THERMAL ANALYSIS The finite element software ANSYS was employed for 2-D thermal analysis. This analysis helped determine

the arrangement of the ground freezing pipes and estimate the time and energy required to form the frozen soil

wall to the design temperature. The analysis algorithm is based on the heat balance equation from the

principle of conservation of energy. Latent heat associated with phase change from non-frozen to frozen state

was taken into consideration. The input data for thermal analysis include geometry, thermal conductivity and enthalpy at different

specific temperature, and boundary conditions. The initial temperature and temperature at boundary of the

model were set at 25°C and the temperature at the locations of the freezing pipes was reduced progressively

from 25°C to -28°C. According to the analysis results as illustrated in Figure 6, the frozen soil wall with

temperature lower than -15°C could be achieved in 20 days of active freezing. The average heat transfer rate

obtained from the analysis was 435kJ/hour·m2. The refrigeration system was required to have a heat exchange

capacity of at least 435kJ/hour·m2 multiplied by total contact area and a factor of 1.3 to account for heat loss.

Figure 6: Temperature field of active freezing from ANSYS analysis

To simulate the effect of pipe extraction to the overall temperature of the frozen soil block, the action of

circulating warm water to defrost the freezing pipe was modeled. Due to heat diffusion, the defrosted region

was frozen again and returned to -15°C by the surrounding frozen soil as illustrated in below Figure 7.

Figure 7: Temperature field of freezing pipe extraction from ANSYS analysis

7 STRESS ANALYSIS, FROST HEAVE AND THAW CONSOLIDATION CONSIDERATION According to the TBM break-through sequence, a local 5m diameter opening (soft eye zone) would be made

on the diaphragm wall of the launching shaft. In advance, a 2.5 thick frozen soil wall that had reached the

design temperature and achieved the target supporting strength had to be formed immediately outside the

opening. After the soft eye zone was broken off manually, the soil and hydrostatic pressure would be retained

by the frozen soil wall spanning across the opening. Analysis was carried out using PLAXIS 2D. The frozen

soil was modelled using Mohr Coulomb’s failure criteria with undrained shear strength Su taken as 0.5 x creep

compressive strength. With a factor of safety of 2 applied, the undrained shear strength of frozen marine

deposits and frozen alluvium were taken respectively as 250kPa and 437.5kPa. Two analysis cases using two

different sets of Young’s modulus had been carried out with one considering the short term loading effect

using E50 and another one considering the long term loading effect using creep modulus. In view of the

at Day 10 of Active Freezing at Day 20 of Active Freezing

immediate after pipe extraction 10 hr after pipe extraction 24 hr after pipe extraction

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relative large frost heave and thaw consolidation ratios, the ground movement and associated impact to the

existing building structures including their foundations were also assessed using PLAXIS 2D. To consider the

effect of frost heave, a positive volume strain was assigned to the frozen zone to model the expansion effect

caused by formation of ice lenses. To mitigate excess settlement caused during the thawing process,

permeation grouting through the grout ports pre-installed in the TBM tunnel lining was proposed. A negative

volume strain was thus assigned to the frozen zone untreated by permeation grouting to simulate the thaw

consolidation. Models for these analyses are shown in Figure 8.

Figure 8: Stress, Frost Heave and Thaw Consolidation Analysis

8 CONCLUSION

It was the first time in Hong Kong to employ AGF using brine as ground improvement method for TBM

launching. The application in Contract No. DC/2009/05 has proven that AGF can be used to strengthen

various types of soil provided that it is supported by careful planning, comprehensive laboratory testing and

rigorous design. In the detailed design, thermal and mechanical properties of the soil including the frost heave

and thaw consolidation were all considered. The TBM was launched successfully in January 2011 and this has

set a benchmark of using AGF to improve the in-situ ground for TBM launching in Hong Kong.

ACKNOWLEDGEMENTS

This paper is published with the kind permission of the Drainage Services Department, the Government of the

Hong Kong Special Administrative Region.

REFERENCES Department of Coal Industry, PRC 1996. Standard for Coal Mining Industry MT/T593-1996.

Harris, J. S. 1995. Ground Freezing in Practice. Thomas Telford, London.

Huang, Z. H., Hu, X. D., Wang, J. Y., Lin, H. B. & Yu, R. B. 2008. Key Techniques in Cross Passage

Construction of Shanghai Yangtze River Tunnel by Artificial Ground Freezing Method. In R. Huang (ed),

The Shanghai Yangtze River Tunnel – Theory, Design and Construction, pp. 205-210. Taylor & Francis

Group, London.

Mitchell, J. M. & Jardine, F. M. 2002. A Guide to Ground Treatment CIRIA C573.

National Administration of Quality and Technology, PRC and Department of Construction, PRC 1999.

Standard for Soil Test Method GB/T 50123-1999.

Shanghai Railway Transport Research Company Ltd. 2006. Technical Code for Cross-passage Freezing

Method DG/TJ08-902/2006.

Storry, R. B., Kitzis, B., Martin, O., Harris, D. & Stenning, A. 2006. Ground Freezing for Cross Passage

Construction beneath an Environmentally Sensitive Area. In Proceedings of HKIE Geotechnical Division

26th Annual Seminar. Hong Kong, 12 May 2006, pp. 161-168

ANSYS, Inc. 2009. ANASYS Thermal Analysis Guide Release 12.1.

Stress Analysis Frost Heave

and Thaw

Consolidation

Analysis